CCE Substructure for Ceramics 4 — Questions and Answers
Question 1: Which casting defect in a metal substructure is caused by gas entrapment during solidification?
- Suck-back porosity
- Microporosity
- Shrink-spot porosity
- Gas porosity (Correct answer)
Correct answer: Gas porosity
Gas porosity results from dissolved or entrapped gases released during metal solidification, leaving voids within the casting.
Question 2: A full-contour zirconia substructure (monolithic) differs from a veneered zirconia substructure in that it:
- Requires a metal core for strength
- Has no veneering ceramic and relies on zirconia's intrinsic color (Correct answer)
- Uses feldspathic porcelain fired over the full surface
- Must be thinner than 0.5 mm
Correct answer: Has no veneering ceramic and relies on zirconia's intrinsic color
Monolithic full-contour zirconia is milled to final shape and stained/glazed without a separate veneering ceramic layer, eliminating chipping risk.
Question 3: When adjusting an occlusal contact on a fired PFM crown, what instrument is preferred to avoid damaging the ceramic surface?
- Coarse sandpaper disc
- Fine diamond bur with water cooling (Correct answer)
- Steel bur at high speed
- Silicon carbide stone dry
Correct answer: Fine diamond bur with water cooling
Fine diamond burs used with water coolant minimize heat generation and micro-crack propagation in the ceramic during occlusal adjustment.
Question 4: In the lost-wax casting technique for metal substructures, which material is used to create the investment mold?
- Gypsum-bonded or phosphate-bonded investment (Correct answer)
- Plaster of Paris
- Silicone rubber
- Dental stone type IV
Correct answer: Gypsum-bonded or phosphate-bonded investment
Phosphate-bonded or gypsum-bonded investments are used to create high-temperature molds for casting metal copings via the lost-wax process.
Question 5: The process of 'airborne particle abrasion' on a completed metal coping before porcelain application serves to:
- Polish the metal surface
- Remove oxides and create micro-retention (Correct answer)
- Increase metal thickness
- Apply a silane coupling agent
Correct answer: Remove oxides and create micro-retention
Particle abrasion removes excess surface oxides and creates microscopic roughness that enhances the mechanical retention of opaque porcelain.
Question 6: What is the purpose of the 'liner' or 'wash' layer sometimes applied to zirconia before veneering ceramic?
- It acts as a colorant
- It improves chemical bonding between zirconia and veneering ceramic (Correct answer)
- It increases zirconia hardness
- It seals surface cracks in zirconia
Correct answer: It improves chemical bonding between zirconia and veneering ceramic
A ceramic liner or interface material applied to zirconia can improve chemical bonding compatibility between the core and the veneering porcelain.
Question 7: Which of the following metal substructure design errors most commonly leads to fracture of the veneering ceramic?
- Broad, flat support areas under ceramic
- Sharp angles and abrupt contour changes in the coping (Correct answer)
- Uniform metal thickness throughout
- Adequate occlusal reduction space
Correct answer: Sharp angles and abrupt contour changes in the coping
Sharp angles and abrupt transitions in the metal coping create stress concentrations in the overlying ceramic, leading to fracture.
CCE Substructure for Ceramics 4
Which casting defect in a metal substructure is caused by gas entrapment during solidification?